Lithium is well suited for use in electrochemical cells because of its high voltage capability and high output density. In miniature cells such as a button cell, a coin cell or other so called "flat cells" a ferrous metal such as stainless steel is commonly used for the container. This is because stainless steel is generally corrosion resistant, is easily formed or machined into a miniature container and is electrically conductive so the container itself can form one terminal of the cell.
In certain flat cell constructions, the second cell terminal is a pin which extends through a hermetically sealed opening in the cell wall. Miniature cells of this type are often constructed so that the cell components such as the anode, cathode and electrolyte are arranged in substantially flat superimposed layers. As used herein, the anode or negative electrode describes the cell material which is oxidized and the cathode or positive electrode includes the cell material which is reduced during cell discharge.
A typical 1.5 to 4 volt miniature cell of this type which measures less than 0.5 inch in diameter and less than about 0.2 inch thick has as its anode, a lithium metal wafer about 0.02 inch thick. The cathode of the cell would vary in thickness depending upon its type and density. For example, a typical cathode which comprises a pressed powder mixture of a conductor, a binder, and an active cathode material such as manganese dioxide (MnO.sub.2), would be about 0.06 inch thick. The cathode and anode are separated within the cell by a separator member which also serves as an electrolyte carrier. Either the anode or cathode is laid flat against the bottom of the container (usually made of a ferrous metal such as stainless steel) while the other is connected to the terminal pin extending through a hermetically sealed opening of the container wall.
Preferably the opening about the pin is sealed by a fused glass-to-metal (GTM) seal. The glass member not only hermetically seals the opening but also electronically insulates the pin from the adjacent ferrous metal container wall.
The GTM seal has been a problem area in cell construction, particularly in high voltage cell systems which utilize lithium as the negative electrode. For example, it has been found that during storage, a conductive corrosive deposit grows from the anode across the surface of the GTM toward the cathode until eventually the glass seal is bridged and the cell shorts.
Efforts to prevent premature failure of the cell and thereby prolong the shelf life of a lithium cell have concentrated primarily on the GTM hermetic seal and/or effective coatings for the seal.
For example, Sandia Report #83-2314 of September, 1984, "Glass Corrosion in Liquid Lithium", suggests that certain glass compositions are better able to withstand corrosion by liquid lithium than others. In U.S. Pat. No. 4,168,351, corosion of a GTM seal is retarded by coating the entire glass surface exposed to the interior of the cell with a protective material such as a metal oxide, polyolefin or fluorocarbon polymer. In U.S. Pat. No. 4,233,372, an inert polymeric coating is applied over the glass surface exposed to the cell environment to reduce chemical attack on the glass and in European Pat. No. 35,074 the exposed glass surface is protected by a silicone layer. A still further solution to the problem of glass corrosion is proposed by U.S. Pat. No. 4,308,323 wherein the resistance of the glass to chemical attack is improved by a graded GTM seal composed of one glass composition bonded to the terminal pin and another glass composition bonded to the wall of the container.
In the present invention, the shelf life of the cell is prolonged regardless of the composition of the glass in the GTM seal or the make-up of the seal. This is accomplished by controlling both the materials of construction of cell components and the arrangement of these components within the cell.
Accordingly, the invention is an improved cell construction for hermetically sealed cells of the type having a terminal pin extending through a wall of the cell housing and electrically insulated from the wall by a GTM seal. While the invention is applicable to a variety of cell configurations and voltages, it is preferred in high voltage cells having an open circuit voltage in the range of about 2.0 volts and above.